1. Field of the Invention
This invention relates to methods of fabricating single crystal silicon MEMS devices and more specifically to methods of fabricating MEMS devices that are suspended on a silicon-on-insulator (SOI) wafer structure.
2. Description of the Related Art
As the name suggests, MEMS devices are very small electromechanical devices such as switches, capacitors, accelerometers, inductors, capacitive sensors and mechanical actuators that combine many of the most desirable aspects of conventional mechanical and solid-state devices. Unlike conventional mechanical devices, MEMS devices can be monolithically integrated with integrated circuitry while providing much improved insertion loss and electrical isolation over solid-state devices. Typically, the MEMS devices are anchored to and suspended above the substrate so that they can move. For example switches open and close, variable capacitors are trimmed or tuned, actuators move back-and-forth and accelerometers deflect.
Oftentimes these devices perform multiple functions or are simultaneously subjected to more than one signal. For example, low frequency signals are used to open and close MEMS switches and trim or tune variable capacitors while they conduct a high frequency AC signal. Mechanical actuators respond to an electrostatic force produced by a low frequency signal while functioning as an actuator. Accelerometers deflect in response to acceleration forces and in turn modulate an AC signal.
In each of these cases, known MEMS devices are designed to balance the requirements of both functions or signals to improve overall performance. Oftentimes this means that the individual performance of one or more aspects of the device must be sacrificed somewhat. For example, Darrin J. Young and Bernhard E. Boser, "A Micromachined Variable Capacitor for Monolithic Low-Noise VCOS," Technical Digest of the 1996 Solid-State Sensor and Actuator Workshop, Hilton Head, S.C., pp. 86-89, 1996 discloses an aluminum micromachined variable capacitor for use as the tuning element in a voltage-controlled oscillator (VCO). To accurately trim the capacitor you want the surface area to be large and the spring constant to be small. However, to make the capacitor insensitive to fluctuations in the signal voltage you want the surface to be small and the spring constant to be large.
Surface micromachining, modified surface micromachining and frontside silicon-on-insulator (SOI) techniques are among those used to fabricate suspended MEMS devices. Surface micromachining uses standard deposition and patterning techniques to "build-up" the MEMS device on a substrate. Young and Boser's paper, U.S. Pat. No. 5,578,976 entitled "Micro Electromechanical RF switch" and M. A. Gretillat et al. "Electrostatic Polysilicon Microrelays Integrated with MOSFETs," proceedings of Micro Electro Mechanical Systems Workshop, 1994, pages 97-101 describe different MEMS switches that use surface micromachining techniques.
K. Petersen, "Micromechanical Membrane switches on Silicon", IBM Journal of Research and Development, Vol. 23, 1979, pages 376-385 describes a modified surface micromachining technique for fabricating MEMS switches. Deposition and patterning techniques are used to build up the device on the surface of a substrate. The device is then released, i.e. suspended, by etching into the surface of the substrate.
Known SOI techniques etch into the top silicon layer to define the device structure. A frontside hydrofluoric (HF) acid etch removes portions of the buried oxide layer to release the devices from the semiconducting substrate. The frontside HF acid etch is a fast low cost technique for releasing the MEMS devices. However, HF acid etching can cause the suspended structure to stick to the substrate thereby ruining the device. Furthermore, the monolithically integrated circuit can exhibit significant parasitic losses from high frequency AC signals that are coupled through the substrate between devices.